For many years, attempts have been made and processes have been employed for electroplating a bright, level zinc-nickel alloy on a substrate such as a metal. Most of the processes employed commercially have employed acid baths, although some have employed alkaline baths. A great variety of additives have been used in attempts to enhance the brightness, levelness, ductility, strength and nickel content of the deposited zinc-nickel alloys.
Electrodeposited (ED) zinc-nickel alloys have found increasing use as corrosion resistant functional coatings. Variations of ED zinc-nickel alloys employing additional alloying elements have been proposed to help achieve specific niche application improvements such as the use of iron to improve paint receptivity, cobalt to improve corrosion resistance, cadmium to decrease hydrogen permeation. However, all the ED zinc-nickel alloys have difficulty with obtaining or retaining desirable mechanical properties. Many such ED zinc-nickel alloys exhibit undesirable characteristics such as cracking, flaking, chipping, brittleness, or low ductility. These undesirable characteristics are believed to be due to the fact that ED zinc-nickel alloys can, and usually do, include crystallographic phases that result in such undesirable characteristics. These crystallographic phases include, for example, the intermetallic ZnNi ‘delta’ phase at a nickel content of about 10 atomic percent (at %), the brass like “gamma” phase at a nickel content of about 12 at %, or the ‘beta’ phase at a nickel content of about 20 at %. Zinc-nickel alloys with all of these phases have been reported by various investigators. Even when the overall nickel content is outside the range normally required to form these phases, it has been reported that these problematic phases may be found in fresh ED zinc-nickel alloy or that they may form, over time, within a matrix of hexagonal zinc containing dissolved nickel.
A continuing and long-felt need has existed in the art for zinc-nickel alloys having enhanced brightness, levelness, ductility and strength, while avoiding the undesirable characteristics associated with previously attempted zinc-nickel alloys including additional alloying elements.